With photographic images made up of silver, the ratio of optical density of the image to a per unit area amount of image-making silver is termed "covering power". This term is employed as a yardstick for estimating optical efficiency of the image-forming silver. As a general rule, the covering power of a silver halide photographic light-sensitive layer is greater the smaller the size of silver halide grains becomes, and becomes lower the larger the size of the silver halide grains. On the other hand, a photographic light-sensitive material of high sensitivity comprises silver halide emulsions which have a large grain size, since sensitivity of a silver halide emulsion layer becomes generally higher the greater a size of silver halide grains is. Consequently, a photographic light-sensitive material of high sensitivity requires a larger per unit area amount of silver in order to attain a definite height of image density. That is to say, it becomes necessary to incorporate a much larger per unit area amount of silver salt into a photographic light-sensitive material in order to achieve both high sensitivity and desired maximum image density.
One attempt to improve covering power while maintaining high sensitivity involves using various kinds of polymers with coarse-grained silver halide emulsions having high sensitivity, as described in British Pat. Nos. 1,048,057 and 1,039,471, and U.S. Pat. Nos. 3,043,697 and 3,446,618. However, these attempts have an insufficient effect on heightening covering power and weaken the emulsion coats. In particular, when photographic light-sensitive materials provided with emulsion coats having strength weakened by application of such techniques are processed in a currently employed automatic developing machine, such techniques have a counterbalancing disadvantage. More specifically, these techniques cause stains on photographic images due to gelatin which has partially been eluted from weakened emulsion coats into the developing or the fixing solution used. The gelatin sticks to the carrying roller installed in the developing machine and is transferred from the carrying roller onto other photographic light-sensitive materials.
As another attempt to obtain photographic images having both high contrast and high covering power with high photographic speed, there are proposed in U.S. Pat. Nos. 2,996,382 and 3,178,282 silver halide photographic light-sensitive materials which contain a coarse-grained silver halide emulsion having high surface sensitivity and a fine-grained silver halide emulsion having fogging nuclei inside the grains in the same layer or in separate layers, which are adjacent to each other.
However, such photographic materials suffer from the defect that development fog tends to be caused therein. Additionally, when those photographic materials are passed through a fixing bath directly after passage through a developing bath without being passed through a stop bath by the use of, e.g., an automatic developing machine or so on, generation of uneven stain comes into question.
Incidentally, as a method for eliminating fog is described in U.S. Pat. No. 3,397,987. The patent discloses a technique wherein heterocyclic nitrogen-containing compounds having a mercapto functional group (e.g., mercaptoazoles) are caused to be held by adsorption on the surface of internally fogged nuclei. However, the uneven stain produced when photographic materials are processed without using any stop bath cannot be reduced with this technique.
Automatic developing machines are now extensively used. Accordingly, it has been increasingly important to prevent the above-described uneven stains.